In digital wireless communication systems, such as digital TV broadcasting systems, electromagnetic waves radiated from the transmitter can arrive at the receiver following multiple paths, with each path resulting in a copy of the transmitted signal with possibly different time delay, phase distortion and energy attenuation. This is the so-called multipath phenomenon.
There are several digital TV standards developed and deployed in the world. Among these are the DVB-T standard used in Europe and most countries in Asia, Africa, Australia and New Zealand, and the ISDB-T standard used in Japan and Brazil. Both standards use OFDM as the modulation and multiplexing scheme in the physical layer due to the relative robustness of OFDM to the so-called multipath phenomenon. In OFDM, the transmitted data is coded in a sequence of symbols that are modulated onto a large number of adjacent orthogonal carriers. OFDM modulation is described, for example, in “Data transmission by frequency division multiplexing using the discrete Fourier transform”, Weinstein, S. B et al. IEEE Transactions on Communication Technology Vol. COM-19, No. 15, October 1971, the contents of which are herein incorporated by reference.
The effect of the multipath phenomenon in OFDM can be seen in the channel impulse response (CIR), which has a positive maximum delay spread. Multipath effects result in inter-symbol interference (ISI), which is a major problem in wireless communication systems, particularly with regard to synchronization.
There are two different OFDM modes: the burst packet mode and the continuous mode. In the burst packet mode, a received signal burst may arrive at the receiver at any time, and the burst may have varying number of OFDM symbols. Systems using this mode include ITU 802.11a and 802.16. Typically in this mode, a training signal (preamble) is transmitted at the beginning of each signal burst. Symbol synchronization is done using this preamble. In the continuous mode, OFDM symbols are transmitted continuously and regularly, but usually no preamble is provided for synchronization purposes. Systems in this mode include DVB-T and ISDB-T.
In order to address the synchronization problem in the continuous mode, the symbols, which are transmitted as a sequence of samples, are separated by a guard period (GP). Each OFDM symbol consists of a guard period, which usually is a cyclic prefix CP, followed by a so-called FFT part. The samples in the GP are identical to the samples in the tail end of the FFT part. The cyclic prefix is described in the paper “On Synchronization in OFDM Systems Using the Cyclic Prefix”, Proceedings of Radio Vetenskaplig Konferens (REVK '96), pp. 663-667, Lulea, Sweden, June 1996, the contents of which are herein incorporated by reference.
It is well known that in OFDM, if the GP is longer than the CIR (channel impulse response) maximum delay spread, and if the symbol synchronization method provides a symbol start point within some tolerable range, then there will be no ISI (inter symbol interference) in the receiver, i.e., the FFT part of each symbol is free of ISI. When referring herein to ISI, we mean ISI in the FFT part of an OFDM symbol. However, OFDM is highly sensitive to symbol synchronization error. The expression ISI-free region is used herein to refer to the aforementioned tolerable range for the symbol start point that results in no ISI in the FFT part of the symbol.
An important feature of digital TV broadcasting is the use of Single Frequency Network (SFN) as a typical network scheme. In a SFN, neighboring broadcast stations broadcast the same signal simultaneously, which can help to improve receiver signal strength. However, SFN generates an artificial multipath effect, which may result in a CIR maximum delay spread much longer than the GP. In this case, ISI is unavoidable, and the symbol synchronization method requires a more accurate symbol start point to minimize ISI.
Symbol synchronization methods can be performed in the time domain or frequency domain. The frequency domain synchronization is performed after FFT operation. A common scheme is to use a time domain synchronization method to do coarse symbol synchronization, and use a frequency domain synchronization method to do fine symbol synchronization.
Conventional algorithms for the symbol synchronization in time domain are MLE (Maximum Likelihood Estimation) using the cyclic prefix of the OFDM symbols as described in US2001622633B1. This method achieves ideal performance only in the AWGN (Additive White Noise Gaussian) channel. To further improve the MLE estimation performance in the AWGN channel, methods using averaging over multiple OFDM symbols and filtering of the estimation results have been proposed. However, when the channel condition becomes severely degraded, data in the GP is badly contaminated by ISI, which leads to significant fluctuation of the synchronization output. To improve the performance of MLE, some schemes have been proposed utilizing GP and pilot tones (US20070098096A1) and GP and PN (pseudo-random) sequence (US20070036234A1), GP and training symbols. These methods perform better under multipath channel conditions. However, a fluctuation still exists because of the ISI contamination in GP and the limited number of pilot tones used for estimation. To mitigate the fluctuation, U.S. Pat. No. 5,732,113A1 uses specially designed training symbols in the time domain to define timing functions.
Chorng-Ren Sheu and Chia-Chi Huang, “A Novel Guard Interval based ISI Free Sampling Region Detection Method for OFDM Systems”, Proceedings of IEEE Vehicular Technology Conference, September 2004, describes a scheme for estimating ISI-free boundary using the characteristics of the guard period and combining the techniques of the delay conjugate multiplication module, phase differential operation, symbol-by-symbol averaging and edge detection. According to the authors, this method guarantees the estimated OFDM start position within the ISI-free region and avoids the negative effect of ISI.
However, the current OFDM symbol synchronization methods are not accurate in multipath channel conditions.